Log-frequency isocontours within and around myelo- and cytoarchitectonically-defined auditory core (thick black lines), reconstructed from electrophysiological recording data reported in Figure 2A of . (See Methods for details on contour rendering). Thin dotted lines show shape of underlying coronal sections of exposed temporal plane and superior temporal gyrus; thick dashed line is estimate of A1/R border. from Morel et al. was chosen for having the most extensive set of recording data over A1 and R, and for being representative of other datasets in Morel et al. and in other combined physiology/cytoarchitectonic experiments.
Tonotopic progressions in macaque can generally be described as follows: wrapping around the caudal cap of the core is a corona of high best-frequency (BF) neurons, with highest BF neurons near or slightly outside the core proper. This high-BF region tends to extend more than halfway along the medial edge of the core (moving rostrally), with BFs then dropping to mid- or low frequencies. Continuing along the medial edge toward the rostralmost tip of core, there is often a moderate increase from low to medium BFs. Progressing posteromedially to anterolaterally across core, there is a steep drop from higher to lower BFs ending in a low BF rostrolateral trough. Along the lateral edge of core, moving posterior to anterior, there is a steeper descent in BF that joins the low-BF trough. Finally, in some cases there is an increase from low-to-medium/high BFs moving out from the anterolateral edge of the less densely myelinated aspect of core (for an almost complete low-to-mid BF gradient in R, see Figure 2B of ).

(a, left panel) Relaxation rate (R₁ sec⁻¹) as function of cortical depth, averaged within probabilistically defined subdivisions of Brodmann’s area 41 (TE1.0, TE1.1, & TE1.2 according to ). Average R₁ within TE1.0 (putative auditory core) decreases steeply from the gray/white boundary (depth fraction 0.0) to a tilted plateau at middle depths (0.3 to 0.6), then again drops steeply at superficial depths (0.7 to 1.0) (error bars: +/−1 SEM over subjects). R₁ within lateral (TE1.1) and medial (TE1.2) subdivisions shows a more gentle monotonic decrease from deep to superficial cortex. For comparison, the left inset is a myelin-stained section of human auditory core and belt cortex (from , contrast reversed) with a similar profile of myelination. (b, right panel): Group spherical average R₁ values sampled at 50% of cortical depth and projected onto a single subject’s left and right inflated hemispheric surfaces. The auditory core is visible in both hemispheres as a keyhole-shaped hyperintensity maximum running posteromedially to anterolaterally over the medial half of Heschl’s gyrus. Hyperintensity maxima can also be observed within the densely myelinated pre- and post-central gyri.

Group average R₁ values from 50% of cortical depth, projected onto the pial surface of the digitally resected temporal lobes of a single subject; (a) local increases in R₁ values along medial Heschl’s gyrus, averaged across both scans, same data as ; (b, c) single-scan R₁ averages show excellent scan-rescan reproducibility; (d) maps of the change in R₁ after removing R₁ variance accounted for by local curvature and thickness show a very similar topology to raw R₁ maps; (e) probability maps of cytoarchitectonically defined TE1.0 (‘core’), TE1.1, and TE1.2. derived from Note that the overlap between probability distributions for TE1.0/TE1.1 and TE1.0/TE1.2 causes some probability maxima for TE1.1 (medial) and TE1.2 (lateral) to be darker colored, as shown in the overlapping probability ovoids at right.

Montage of individual participants’ R₁ maps, with values sampled halfway through cortex and projected onto the inflated surface of temporal and frontal lobes. As shown in the colorbars, the range of projected R₁ values is constant over participants, with the colorscale slope adjusted slightly to show individual patterns of myelination. Keyhole-shaped regional increases in R₁ oriented posteromedially to anterolaterally across Heschl’s gyrus were observed in all hemispheres (medialmost aspect indicated by arrow head) except the left hemisphere of participant S4 (marked with black diamond shape). R₁ maxima are also observed along the pre- and post-central gyri within strongly myelinated presumptive primary motor and somatosensory regions. Abbreviations: S. Circular S => superior circular sulcus of the insula; Cent S => central sulcus; Heschl’s G => Heschl’s gyrus.

Combined tonotopic maps, gradients and R₁ contours from group averages. Colormap shows fMRI results for characteristic frequency with logarithmic scaling in Hz around the color wheel. The angle of the overlaid white arrows shows the tonotopic gradient direction from low-to-high frequency preference at approximately 1/3 octave steps within tonotopic maps (perpendicular to isofrequency contour), with arrow length reflecting the approximate magnification factor (reciprocal of gradient vector). Dots indicate points where the gradient direction was unclear or interrupted. Dashed lines show R₁ values in grayscale-coded steps of 0.005 sec⁻¹ (same data as ); curved yellow line is suggested A1/R border based on presence of low-frequency reversal and R₁ contour. Top and middle inset figures show tonotopic averages with left monaural and binaural stimulation from the same subjects as in the combined average. Bottom inset shows comparison tonotopic averages from different sets of participants and experimental setups; all tonotopic maps were identically masked using the independent auditory localizer (see Methods).

Montage of individual participants’ tonotopic maps and R₁ contours, sampled halfway through cortex and detrended for effects of local curvature and thickness (see Methods, and ). Tonotopic colormaps are as in . For visual clarity, contours indicate stepwise progression of R₁ values in probable auditory core only; see heatscale images in for raw R₁ data for each individual.

Functional tonotopic maps of an individual subject with focus on the right hemisphere of the inflated superior temporal lobe. Top row shows results from three different sessions of tonotopy using bandpass-swept vocalization stimuli (4 runs each); bottom left inset shows one session using bandpass-filtered music (4 runs), bottom right shows one session using bandpass-filtered, amplitude-modulated white noise (4 runs).